1. Field of the Invention
There is presently intense interest in flat panel displays not only to replace the ordinary cathode ray tube but also to go beyond the limits of liquid crystal displays. A flat panel display is one in which the display area is maximized and the operating volume of the device minimized to yield a maximization of display area to volume. An emissive flat panel display is one in which electrons are emitted from the cathode, and then directed to discrete positions on a luminescent screen. The instant invention relates to a greatly improved emissive cathode which combines thermionic emission with a moderately high to high electric field for barrier reduction and field emission in a novel structure that is less expensive to manufacture and more rugged than its existing counterparts. The combination of thermionic emission and a moderate electric field is called Schottky emission. Since the electric fields in this invention go from moderate to high, the emission can greatly surpass Schottky emission.
Furthermore the present invention provides method and apparatus for generation and regeneration of sharp asperities to increase the useful lifetime of the cathode. These asperities (whiskers) are responsible for providing the field emission component of the current. A deficit of extant field emission flat panel displays is that when the asperities lose their sharpness or length (tips become dulled), sufficient emission ceases, the asperity cannot be restored, and the whisker becomes ineffective.
As practiced in the present invention, it is possible to reduce the effective work function by about 1 eV due to the Schottky reduction in barrier height. As is shown in the accompanying tables, about a 1 eV decrease in work function can increase the current density by as much as .about.10.sup.6. The actual increase is even greater than this because Schottky modified the equation for thermionic emission to include only the effect of barrier height reduction by a moderate field. He did not include the effects of tunneling through a barrier that has been appreciably thinned by a high electric field. For a very high electric field, tunneling effects produce an even much higher emission rate; and the effects of combined thermionic emission and field emission are much more complicated than mere Schottky emission.
Whereas, the improved cathode of the immediate invention is presented in the context of flat panel displays, it may be utilized in a number of other applications, with or without the regenerative capability. Such applications comprise devices in which there is an emissive cathode structure for producing electrons. There are clear advantages for the instant invention in the case of a flat panel display which requires a relatively large cathode area, because the present invention avoids excessive power loss due to radiation and conduction loss by permitting operation of the cathode at a significantly lower temperature than if it operated solely as a thermionic emitter. Additionally the moderate to high electric field mitigates against space charge limitations of the current. There are also clear advantages for the present invention over purely field emitting cathodes in a flat panel display: 1) as there is an additional control over the emission current; 2) the effects of asperity tip dulling are mitigated both by regeneration and separate control of emission; 3) expensive processes for making a precisely similar and precisely arranged multitude array of field emitting cathodes are avoided; and 4) the immediate invention results in a more robust cathode than the field emission cathode in which microscopic spacing between anode and cathode and its maintenance is critical.